1. Field of Invention
The present invention relates to catalyst component supported on magnesium-based medium, methods of preparing the catalyst component, and methods of using the catalyst component in a catalyst system to produce polyolefin, particularly polyethylene, more particularly linear low density polyethylene. More specifically, the present invention relates to a highly active catalyst system capable of producing ethylene/alpha-olefins copolymers, particularly, linear low-density polyethylene, having narrow molecular weight distribution, narrow (or improved) branching compositional distribution, and reduced lower molecular weight component content. Also, the present invention relates to a catalytic process using the catalyst system for producing linear low density polyethylene having good powder flowability, has a high bulk density, and contains a small amount of fine powders.
2. Description of the Related Art
Linear low density polyethylene polymers (LLDPE) have a density of about 0.900 g/cc to about 0.945 g/cc. Preferably, LLDPE has a density that is less than 0.930 g/cc. LLDPE, when compared to other polyethylene polymers, such as homopolymers of polyethylene, possesses advantageous properties. Such properties are described in various references, including U.S. Pat. No. 4,076,698 by Anderson, et al. LLDPE has rapidly increasing commercial importance in commodity and industrial applications including blown and cast films, injection molding, rotational molding, blow molding, pipe, tubing, and wire and cable manufacturing. Intensive research has been directed to development of high performance LLDPE resins having improved impact strength, higher toughness, higher transparency, less low molecular weight component content, and narrower compositional distributions. The catalyst systems are crucial for producing such LLDPE.
Optimizing the properties of LLDPE by varying product molecular weight, molecular weight distribution (MWD), and density is performed to match the required product specifications. Narrowing the MWD, lowering the density of LLDPE, improving branching compositional distribution, and reducing low molecular weight extractable fraction are desirable as the tear strength, impact strength, puncture resistance, toughness, and clarity properties of films from these resins can be much improved. Although the actual molecular weight of a resin can be adjusted by altering process conditions in copolymerization reactions, MWD, density, branching compositional distribution, and low molecular weight extractable fraction of a LLDPE resin are strongly influenced by catalyst composition.
Several catalyst systems have been examined to manufacture LLDPE. Chromium containing catalysts have been examined. Single site catalyst systems employing organometallic compounds and aluminoxane can provide improved control of MWD and branching compositional distributions compared to traditional Ziegler-Natta catalyst systems. To use single site catalyst systems in current industrial scale gas phase processes, the catalyst components are immobilized on a carrier or support, such as silica or alumina. Using supported or heterogeneous catalysts improves reactor operability and ease of handling and increases process efficiencies by forming polymeric particles that have a desirable morphology and density. However, the solubility of organometallic compounds and cocatalysts such as methylaluminoxane (MAO) requires immobilization processes on inorganic supports in systems that are costly. Accordingly, it can be difficult to apply single site catalysts in existing polymerization processes without major process modification and capital investments. So the application of such systems for producing LLDPE has its drawbacks for gas phase processes.
In contrast, advanced Ziegler-Natta catalysts based on supported titanium systems have received recent research interest for producing high performance LLDPE resins, such as Super-Hexene resins. Super-Hexene resins are ethylene/hexene copolymers having narrow molecular weight distributions, uniform compositional distribution, and high performance properties comparable to ethylene-octene copolymers produced by single site catalysts. The advanced Ziegler-Natta catalysts are directly applicable to existing fluidized gas phase processes, without polymerization process modification.
For example, catalysts prepared in-situ by reacting magnesium metal with at least one halogenated hydrocarbon and at least one tetravalent titanium compound have been described. Reacting magnesium metal powder with butyl chloride in a non-polar solvent in the presence of TiCl4/Ti(OR)4 to form a catalyst for gas phase ethylene co-polymerization has been disclosed. An advantage of this synthesis method for preparing Ziegler-Natta catalyst is to form homogeneous active sites and to simplify preparation procedure. However, these catalysts can show broad particle size distribution as well as poor morphology and poor operability for producing lower density resins, and inferior comonomer incorporation. The LLDPE resins obtained using such catalysts do not have the narrow molecular weight distribution and compositional distribution that are desirable for high performance resins. Moreover, the catalyst composition can not produce LLDPE with a density of less than 0.917 at economically favorable production rates because of poor powder flowability. The poor flowability is caused by resin stickiness, chunk formation, and reactor fouling.
Other supported titanium catalyst systems for LLDPE are obtained by dissolving MgCl2 with [TiCl3 (AlCl3)1/3] in tetrahydrofuran (THF) to make a solution containing MgCl2 and titanium halide that is subsequently immobilized on silica support. A process wherein MgCl2 is dissolved in an electron donating solvent and reacted with alkylaluminum compounds to solidify magnesium halide with aluminum alkoxy compounds has also been disclosed. The solid was then contacted with titanium halide to give a solid catalyst with effective co-polymerization ability. However, the preparation of such catalyst systems can require complicated processing steps, and the LLDPE products obtained using these catalyst systems do not possess narrow molecular weight distribution and compositional distribution required for high performance resins. This inadequate molecular weight and compositional distribution presumably exists because of broadly inhomogeneous active sites in such catalyst systems.
Also, a catalyst system in which dialkylmagnesium and silane compounds are reacted with an —OH group on a silica support, which is then contacted with transition metal halide to form a relatively homogeneous active site, has also been disclosed. This silica supported catalyst system exhibits more homogeneous ethylene polymerization or co-polymerization capability than magnesium-based supported titanium halide catalyst systems as measured by resin MWD and compositional distribution. However, such catalyst systems require extra processing steps because the silica support must be treated, either chemically or thermally, to remove bound water and excess —OH groups prior to the formation of the catalyst.
Additionally, catalyst systems in which dialkylmagnesium compounds are impregnated into a silica support containing —OH groups to form a first reaction product have been disclosed. The first reaction product is then halogenated with HCl to convert the organomagnesium derived compound to MgCl2 thereby forming a second reaction product. The second reaction product is then treated with a transition metal halide such as TiCl4, a particular type of electron donor, and at least one Group 2 or 13 organometallic compound such as diethylaluminum chloride. The multi-step process of this catalyst preparation is complicated and is a difficult process to use to provide controlled, stable catalyst quality.
Other art describes a silica supported Ziegler-Natta catalyst system for ethylene polymerization using substituted pyridines as electron donors. The catalyst is the reaction product of a tetravalent titanium halide with —OH group of silica support in the presence of the electron donor. However, the catalyst system is not suitable for producing linear low density polyethylene, because the system has poor comonomer response and branching compositional distribution.
In the prior art, the preparation methods of Ziegler-Natta catalysts described for the catalytic control of molecular weight distribution and/or branching compositional distribution are directed toward complicated tuning art of controlling the active site formation process, which in turn requires careful control of the catalyst precipitation process to ensure consistent catalyst properties and the formation of uniform catalyst active sites. Deteriorated catalyst properties are often present in the absence of control over the precipitation process, especially in multi-step processes.
Therefore, there is a need for a catalyst composition, more especially an advanced Ziegler-Natta alpha-olefin copolymerization catalyst having superior performance including comonomer incorporation, comonomer composition distribution, and molecular weight distribution. It is desirable to devise a magnesium-based catalyst system to produce LLDPE resins of narrow molecular weight distribution and of lower density without reactor fouling at high production rates. It is also desirable to provide a supported catalyst system with high catalyst efficiency to control desired morphology, bulk density, and kinetic characteristics. The supported catalyst system should also minimize resin stickiness, chunk formation, and reactor fouling in the fluid bed gas-phase process. Chunk formation and powder stickiness cause various troubles of operations, and consequently significantly decrease operation efficiency. Therefore, it is desirable to decrease the amount of adhesive materials and chunks. It is further desirable to have a catalyst system that can be prepared by a simple and efficient process.